Terry Pratchett's controversial new documentary, Choosing to Die, ends in a man's death. Peter Smedley had motor neurone disease and chose to go to the Dignitas clinic in Switzerland before he lost the ability to "self-determine" (article 8 of the Human Rights Act upholds for us all the right to self-determination).

In the film, which airs on BBC2 on Monday evening, Smedley ends his life at 71 with a glass of lethal medicine, his wife on one side, a Dignitas helper on the other, Pratchett standing aside, Pratchett's assistant at the door.

I had all the standard reservations about why you'd allow a hoard of near-strangers at your death bed, but that was a failure of my imagination.

Smedley's final minutes are characterised by a sort of bland foreboding, as though he were waiting for a bus to get root canal treatment. He asks his wife what the time is at one point, drumming the table, choosing which chocolate to eat after the final draught (it tastes disgusting, as a lethal poison might).

"Not that I'm in a hurry," he says mildly; there's an unspoken "but we may as well get this over with …"

The helper asks him, "Are you sure you want to drink this medicament with which you will sleep and die?" He drinks it, and it's done. He calls for water, she holds his head: "No more water." Seconds later, his breathing grows heavy, very shortly he is dead.

It is not the first time a death has been shown on television – Emma Swain, acting controller of knowledge at the BBC, says BBC1 has done it before, as has Sky and an Irish channel – but nevertheless, I've now seen a man whom I didn't know die. I don't see how this can be anything other than a Rubicon moment. I don't necessarily mean that in a bad way.

Pratchett is terribly affected by the experience, saying into the camera that he doesn't know if he would be able to do it himself.His dilemma as someone with Alzheimer's disease is a terrible one, of course, since with Alzheimer's more than any other condition, the decision to die has to be taken prematurely. By the time he's in enough anguish to want death, nobody will listen to him.

In Pratchett's film, though, the process of chosen suicide looks human and beautiful – to me, anyway. Not everybody at the screening agreed though: a reporter from the Daily Mail said, quite angrily, to Swain: "He was calling for water, his breathing was laboured. I don't think it was peaceful."

I think that's a charge to lay against death rather than suicide, however. When people talk about a "peaceful death", they mean it in the same way as "beautiful birth", with the proviso that you already understand how incredibly fucking painful it's going to be, whether you're going hence or coming hither.

Geoff Morris, who campaigns for Care Not Killing, and himself has MS, made a persuasive and moving case against the film, which you could argue – he certainly would – should have been videoed and shown as a companion piece. "You come out of it thinking, 'Aren't they champions? Shouldn't they be given medals?' It was a complete failure to put the other side of the argument."

Swain defended it by saying that a) it was an authored piece, not a debate, and b) there would be a feature-length Newsnight discussion afterwards. This sounded eminently reasonable but a bit unequal.

"At this point in time," Morris continued, "when the government is devaluing disabled people the way it is at the moment, you make this film that says we should consider whether we should be here. We should consider the impediment we are to society."

I'm thrown back to the other man who dies in this film, 42-year-old Andrew Colgan who, distressingly, doesn't even seem all that ill when he chooses to die. If there's one message his death forces on you, it's that nobody can tell anybody else how ill they feel, and what their quality of life is.

I feel myself coming unstuck from the position of disability campaigners: can't a person with crushing disabilities want to die, without their death devaluing the lives of all other disabled people? Isn't that part of equality, that you can still think and act not as an ambassador, nor a campaigner, nor in the interests of others who share your condition, but just as an individual, weary of the effort?

In the film, Pratchett interviews the widow of a Belgian author who killed himself following an Alzheimer's diagnosis, and says at the end, "I shall remember you." There is so much feeling in his voice: hope, foreboding, determination, denial, self-awareness. It is an awful glimpse into his predicament, and the predicament of mortality.